[callinyouin]

> determining if somebody's going to bump in you

I think the human aspect of that is a huge factor. I've always had a problem being around large crowds (anxiety, dizziness, mental strain). I think my nervous brain is constantly trying to determine how I might interact with the people around me, and when there are so many people I just get overwhelmed. Same thing happened in the (rather large) office I work at. For the first few days I was feeling mental strain only a few hours into the day, but after the office and the people in it became a known quantity I think my brain stopped trying to analyze everything.